1. Field of the Invention
The present invention relates generally to the use of dense phase gases for cleaning substrates. More particularly, the present invention relates to a process utilizing phase shifting of dense phase gases or gas mixtures in order to enhance the cleaning of a wide variety of substrates, including complex materials and hardware.
2. Description of Related Art
Conventional solvent-aided cleaning processes are currently being re-evaluated due to problems with air pollution and ozone depletion. In addition, recent environmental legislation mandates that many of the organic solvents used in these processes be banned or their use severely limited. The use of dense phase gases or gas mixtures for cleaning a wide variety of materials has been under investigation as an alternative to the above-mentioned solvent based cleaning processes. A dense phase gas is a gas compressed to either supercritical or subcritical conditions to achieve liquid-like densities. These dense phase gases or gas mixtures are also referred to as dense fluids. Unlike organic solvents, such as n hexane or 1,1,1 trichloroethane, dense fluids exhibit unique physical and chemical properties such as low surface tension, low viscosity, and variable solute carrying capacity.
The solvent properties of compressed gases is well known. In the late 1800's, Hannay and Hogarth found that inorganic salts could be dissolved in supercritical ethanol and ether (J. B. Hannay and H. Hogarth, J.Proc.Roy.Soc. (London), 29, p. 324, 1897). By the early 1900's, Buchner discovered that the solubility of organics such as naphthalene and phenols in supercritical carbon dioxide increased with pressure (E. A. Buchner, Z.Physik.Chem., 54, p. 665, 1906). Within forty years Francis had established a large solubility database for liquefied carbon dioxide which showed that many organic compounds were completely miscible (A. W. Francis. J.Phys.Chem., 58, p. 1099, 1954).
In the 1960's there was much research and use of dense phase gases in the area of chromatography. Supercritical fluids (SCF) were used as the mobile phase in separating non volatile chemicals (S. R. Springston and M. Novotny, "Kinetic Optimization of Capillary Super-critical Chromatography using Carbon Dioxide as the Mobile Phase", CHROMATOGRAPHIA, Vol. 14, No. 12, p. 679, December 1981). Today the environmental risks and costs associated with conventional solvent aided separation processes require industry to develop safer and more cost-effective alternatives. The volume of current literature on solvent-aided separation processes using dense carbon dioxide as a solvent is evidence of the extent of industrial research and development in the field. Documented industrial applications utilizing dense fluids include extraction of oil from soybeans (J. P. Friedrich and G. R. List and A. J. Heakin, "Petroleum Free Extracts of Oil from Soybeans", JAOCS, Vol. 59, No. 7, July 1982), decaffination of coffee (C. Grimmett, Chem.Ind., Vol. 6, p. 228, 1981), extraction of pyridines from coal (T. G. Squires, et al, "Super-critical Solvents. Carbon Dioxide Extraction of Retained Pyridine from Pyridine Extracts of Coal", FUEL, Vol. 61, November 1982), extraction of flavorants from hops (R. Vollbrecht, "Extraction of Hops with Supercritical Carbon Dioxide", Chemistry and Industry, 19 June 1982), and regenerating absorbents (activated carbon) (M. Modell, "Process for Regenerating Absorbents with Supercritical Fluids", U.S. Pat. No. 4,124,528, 7 November 1978).
Electro-optical devices, lasers and spacecraft assemblies are fabricated from many different types of materials having various internal and external geometrical structures which are generally contaminated with more than one type of contamination. These highly complex and delicate assemblies can be classified together as "complex hardware". Conventional cleaning techniques for removing contamination from complex hardware require cleaning at each stage of assembly. In addition to the above-mentioned problems with conventional solvent aided cleaning techniques, there is also a problem of recontamination of the complex hardware at any stage during the assembly process. Such recontamination reguires disassembly, cleaning, and reassembly. Accordingly, there is a present need to provide alternative cleaning processes which are suitable for use in removing more than one type of contamination from complex hardware in a single process.